The present invention relates to an airflow rate measuring apparatus of the Karman vortex type which can effectively measure the air intake of an engine, and, more particularly, to an airflow rate measuring apparatus of the Karman vortex type which can effectively measure the weight flow rate rather than the volume flow rate of intake air flowing into an intake pipe.
The airflow rate measuring apparatus of a Karman vortex comprises a vortex generator of a column-like shape set across the inside of a duct in which air to be measured flows. When an airflow is present in the duct, the vortex generator generates vortexes at the downstream side. The airflow rate in the duct is measured utilizing the principle that the vortex generating frequency is proportional to the airflow speed. More specifically, a vortex detection element of the heated wire type is set in a downstream side surface of the vortex generator. The detection element detects the vortex generating frequency so as to calculate the airflow rate in the duct.
Since the resistance component interfering with airflow is relatively small in an airflow rate measuring apparatus of the Karman vortex type as described above, the apparatus can effectively measure the flow rate of the intake air of an engine.
In such an apparatus, operation of the vortex generator is preferably stable so that the vortex generating frequency will be linear to the airflow rate within a wide range of measurement. Ideally, the coefficient of resistance should also be small. In order to satisfy these requirements, various studies have been made as to the shape of the vortex generator and the like. However, a vortex generator satisfying all desired characteristics has not yet been devised.
In this type of apparatus the vortex generating frequency is proportional to the airflow speed. Therefore, the Karman vortex frequency represents the volume flow of air. However, the intake airflow rate measuring apparatus of a combustion engine is required to measure not the volume flow but the weight flow. Therefore, a measurement signal corresponding to the Karman vortex frequency cannot be directly used as an intake airflow rate measurement signal.
In view of the above, in U.S. patent application Ser. No. 739,004, previously proposed by the present applicant, the typical dimension of the surface of a vortex generator facing an airflow is varied in correspondence with the aspect of the airflow to be measured, e.g., air pressure, so that the measurement output reflects a state of air density. In this manner, a Karman vortex frequency is generated in correspondence with the weight airflow rate.
When the typical dimension of a vortex generator is varied in correspondence with the air density, the shape of the vortex generator is limited. For example, a widening effect of the dynamic range of measurement may be limited. Consequently, an improvement in the linearity of the Karman vortex frequency with the flow rate may also be limited.